Abstract : Cytochrome P450 enzymes (CYPs or P450s) are the most important enzymes involved in the phase I metabolism of drugs (and other xenobiotics) in humans, and the corresponding drug metabolites are needed as reference substances for their structural confirmation and for pharmacological or toxicological characterization. We have previously shown that biotechnological synthesis of such metabolites is feasible by whole-cell biotransformation with human CYPs recombinantly expressed in the fission yeast Schizosaccharomyces pombe. It was the aim of this study to compare the activity of seven human microsomal CYPs (CYP2C9, CYP2D6, CYP3A4, CYP3A5, CYP3A7, CYP17, and CYP21) upon coexpression with NADPH-cytochrome P450 oxidoreductases (CPRs) from various origins, namely, human CPR (hCPR) and its homologues from fission yeast (ccr1) and the bishop's weed Ammi majus (AmCPR), respectively. For this purpose, 28 recombinant strains were needed, with five of them having been constructed previously and 23 strains being newly constructed. Bioconversion experiments showed that coexpression of a CPR does not only influence the reaction rate but, in some cases, also exerts an influence on the metabolite pattern. For CYP3A enzymes, coexpression of hCPR yielded the best results, while for another two, hCPR was equally helpful as ccr1 (both CYP17 and CYP21) or AmCPR (CYP17 only), respectively. Interestingly, CYP2D6 displayed its highest activity when coexpressed with ccr1 and CYP2C9 with AmCPR. These results corroborate the view of CPR as a well-suited bio-brick in synthetic biology for the construction of artificial enzyme complexes.